regex: port to non-glibc/lock-using systems

regex: port to non-glibc/lock-using systems

Since 29-05-2013 commit, 55ba71f4, compilation on a non-glibc
system with GNULIB_LOCK would fail due to absence of the
included "glthread/lock.h".  This would affect any package
for which the "lock" module is used only by the regex module,
and not explicitly used.
* m4/regex.m4 (gl_PREREQ_REGEX): Require gl_GLIBC21.
* modules/regex (Depends-on) [!GLIBC && $ac_use_included_regex]:
Add a dependency on the "lock" module.